You should have your custom files in your content folder or debug folder so they gets distributed properly on release. I am not sure what happens if i load a file from a completely different path since there was no reason to do that.
If you really want to load from a path far down you could type "../" to go a map down, depends on where you put the solution how much maps you need to go down to reach the "c:\" but you can see in the debug information where it is looking for the file so that should not be a problem.
But i really advice you to use the proper locations for your custom content or you might end up changing tons when you want to release.